1. Field of the Invention
The present invention relates generally to ceramic moulds for casting articles in casting processes of the lost pattern type. In particular the invention relates to a method of making such moulds and to ceramic slips for use in the method.
2. Summary of the Prior Art
As is already known, lost wax pattern casting processes using a ceramic mould comprise the following steps:
making a pattern, generally of wax, of the article to be produced; PA0 forming a shell mould around the pattern by applying successive coatings to the pattern from a slip of specific composition and drying each coating before application of the next; PA0 heat treating or firing the shell mould following specific temperature and time cycles so as to obtain a strengthening or "baking" of the mould and the elimination of the pattern; PA0 casting the article in the mould thus obtained, which may be in the form of a "cluster" mould for the simultaneous casting of several articles, particularly when casting articles such as engine blades for aeronautical applications, a field with which the invention is particularly concerned; and, PA0 disposal of the shell mould. PA0 (a) providing a wax-type pattern of the article to be cast; PA0 (b) forming a contact layer on said pattern in a known manner; PA0 (c) providing a preparation mould for defining the outer shape of the casting mould which is to be made; PA0 (d) placing the pattern having the contact layer thereon in said preparation mould to define a cavity between said pattern and said preparation mould; PA0 (e) introducing into said cavity a ceramic slip having a composition such that it expands to form a coherent cellular foam which fills said cavity and adheres to the contact layer of said pattern; PA0 (f) removing the pattern and adhered foam from the preparation mould after from 5 to 30 minutes; PA0 (g) eliminating the wax-type pattern to leave a casting mould formed by the contact layer and adhered ceramic foam; and PA0 (h) firing said casting mould to strengthen said mould by sintering. PA0 from 30 to 40% of a binder such as sodium silicate or phosphoric acid; PA0 from 40 to 50% of a refractory ceramic charge containing materials which are known per se, such as zirconium, alumina, mullite, or mixtures thereof; PA0 from 5 to 10% of a product which, in the presence of a catalyst, reacts to cause a release of gas, said product preferably being selected from the group consisting of perborates and peroxides; PA0 from 3 to 4% catalyst, for example polyoxymethylene; and PA0 from 2 to 3% of adjuvants, such as cohesion agents or deflocculents which are known per se. PA0 from 40 to 60% of a refractory ceramic charge containing materials which are known per se, such as zirconium, alumina, mullite or mixtures thereof; PA0 from 2 to 3% of one or more known cohesion agents; and, PA0 two silicone-based products, each present in a proportion of from 20 to 30%, which mix to form a coherent foam by gas release and reticulation of the silicone polymers.
Examples of slip compositions used for the manufacture of shell moulds for casting have been described in particular in EP-A-0 251 847; U.S. Pat. No. 3,249,927, and FR-A-2 348 772.
Also known from FR-A-2 479 044 is a casting process using a fusible pattern obtained at least partly from a foaming composition of polyurethane foam or epoxy foam type.
It is an object of the invention to provide an alternative method for the manufacture of ceramic moulds intended for lost pattern casting applications, so that the moulds obtained meet the various conditions of use generally sought in this field, making it possible to obtain cast articles meeting strict quality standards, particularly for aeronautical applications such as super-alloy components for aero-engines.
A further object of the invention is to provide a method of making the said moulds in which handling is simplified and the risk of rejects during manufacture is reduced, while also saving time in the manufacturing process.